Although representation of hydrology is included in all regional climate models (RCMs), the utility of hydrological results from RCMs varies considerably from model to model. Studies to evaluate and ...compare the hydrological components of a suite of RCMs and their use in assessing hydrological impacts from future climate change were carried out over Europe. This included using different methods to transfer RCM runoff directly to river discharge and coupling different RCMs to offline hydrological models using different methods to transfer the climate change signal between models. The work focused on drainage areas to the Baltic Basin, the Bothnian Bay Basin and the Rhine Basin. A total of 20 anthropogenic climate change scenario simulations from 11 different RCMs were used. One conclusion is that choice of GCM (global climate model) has a larger impact on projected hydrological change than either selection of emissions scenario or RCM used for downscaling.
One of the less known aspects of operational flood forecasting systems in complex topographic areas is the way how the uncertainties of its components propagate and superpose when they are fed into a ...hydrological model. This paper describes an experimental framework for investigating the relative contribution of meteorological forcing uncertainties, initial conditions uncertainties and hydrological model parameter uncertainties in the realization of hydrological ensemble forecasts. Simulations were done for a representative small-scale basin of the Swiss Alps, the Verzasca river basin (186
km
2).
For seven events in the time frame from June 2007 to November 2008 it was possible to quantify the uncertainty for a five-day forecast range yielded by inputs of an ensemble numerical weather prediction (NWP) model (COSMO-LEPS, 16 members), the uncertainty in real-time assimilation of weather radar precipitation fields expressed using an ensemble approach (REAL, 25 members), and the equifinal parameter realizations of the hydrological model adopted (PREVAH, 26 members). Combining the three kinds of uncertainty results in a hydrological ensemble of 10,400 members. Analyses of sub-samples from the ensemble provide insight in the contribution of each kind of uncertainty to the total uncertainty.
The results confirm our expectations and show that for the operational simulation of peak-runoff events the hydrological model uncertainty is less pronounced than the uncertainty obtained by propagating radar precipitation fields (by a factor larger than 4 in our specific setup) and NWP forecasts through the hydrological model (by a factor larger than 10). The use of precipitation radar ensembles for generating ensembles of initial conditions shows that the uncertainty in initial conditions decays within the first 48
hours of the forecast. We also show that the total spread obtained when superposing two or more sources of uncertainty is larger than the cumulated spread of experiments when only one uncertainty source is propagated through the hydrological model. The full spread obtained from uncertainty superposition is growing non-linearly.
► Three uncertainties are superposed and propagated through a hydrological model. ► A) Parameter uncertainty, estimated by Monte Carlo sampling (MOD). ► B) Initial conditions uncertainty, estimated by ensemble weather radar QPE (REAL). ► C) Forecast uncertainty, obtained by a limited-area atmospheric EPS (LEPS). ► LEPS is the main contributor to the total uncertainty, followed by REAL and MOD.
To assess the potential impacts of climate change on the river flows and snow cover duration in the upper Rhine catchment, climate model integrations coupled to a hyrological model are conducted. ...Three 30-year simulations are considered: The first uses large-scale boundary conditions from the ECMWF 40-year reanalysis during the period 1961-1990 and is used for validation of the model chain. The two other simulations are climate change integrations using boundary conditions from a Hadley Centre GCM covering the periods 1961-1990 (control) and 2071-2100 (scenario), using the IPCC SRES A2 greenhouse gas scenario. These three low-resolution data sets are downscaled using the ETH regional climate model (CHRM) with a spatial resolution of 56 km. For hydrological simulations, the Water Flow and Balance Simulation Model (WaSiM) was used. This physically based distributed model with a horizontal gridspacing of 1 km was interfaced with the regional climate model and run with an hourly time step for a total of 20 subcatchments of the Rhine basin, extending from the Alpine catchments down to Cologne and covering an area of 146'000 km2. Analysis and validation of the daily hydrographs and the frequency distributions show that WaSiM is able to realistically reproduce the runoff signal. Also the characteristic phase change between gauges along the Rhine from predominantly nival to pluvial regimes is captured. The results of the climate change integrations show a pronounced decline in mean summer runoff (caused by the mean reduction of precipitation) and a considerable reduction of winter snow cover (caused by the mean warming). These changes also influence the contributed runoff fraction of the Alpine catchments.
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